Help! PA system AC hum

[peter d]

A church recently moved its mixing board to a new location in the sanctuary, so I had to relocate the receptacle for the equipment. I installed the receptacle, using EMT, which then transitions to an old circuit which is fed with AC cable. I did not run a ground wire with the EMT.

Upon power-up, I was getting a nasty AC hum in the main speakers, and an even worse and louder hum in the monitor speakers which are powered by a different amp on a different circuit, and I think an different panel board alltogether.

I unplugged both the monitor amp and disconnected the 1/4" line ins, and the hum in the main speakers decreased noticeably but not terribly.

Now, this problem did not exist before when the mixing board was plugged into a receptacle in the sanctuary, which, although I'm not sure, I think is fed from the same panel board as the monitor amp.

This sounds like a classic ground loop scenario but I have no idea how to correct it.

The sub panel board I fed the new receptacle from is a total rats nest of wires, and I have no idea if it has an improper ground to neutral connection or not. It's fed with EMT which also has no ground wire. The newest wiring in the distribution system in the building is mid '70s and all systems use the EMT for the ground.

This hum is totally unacceptable and now I would like to get the egg off my face even though this is volunteer work.

[ September 14, 2005, 10:34 PM: Message edited by: peter d ]

 

[benaround]

Re: Help! PA system AC hum

Peter,

I'd put my money on cris-crossed neutrals.

 

[LarryFine]

Re: Help! PA system AC hum

I'd start with a process of elimination. Try, as a temporary solution, a 3-prong-to-2-prong adapter to lift the chassis ground from the power supply.

Another temporary troubleshooting method would be to try plugging the offending amp in with an extension cord where the rest of the audio equipment is plugged in.

You may end up having to disconnect the shield of the interconnecting audio cable(s), and you may have to experiment to find which end to cut for best effect.

 

[dereckbc]

Re: Help! PA system AC hum

It's a loop. The best solution is to feed all components (mixers, amps, etc) from one dedicated circuit with a EGC and nothing sharing plugged into the dedicated circuit.

Other solutions are isolation techniques like signal transformers or optical couplers on all lines.

 

[hurk27]

Re: Help! PA system AC hum

Peter go to any professional PA system supply house or guitar store that Carry's PA equipment and ask for a 1/4" 1 to 1 isolating transformer. This will loose the ground loop in a safe way. You might have to get a 1/4" to XLR matching transformer then a XLR to 1/4" to do the same thing, Switch Craft makes many different configurations that can adapt to any audio hook up. but it will work. How are the amps fed with audio, are both the monitors and main amps fed through the mixing board? you will have to disconnect each audio cable that runs between each piece of equipment until you find the offending cable(S) then put the above isolating transformers in-line with it. I would start with the audio cables feeding the amps first.

What causes the hum is the grounding between two pieces of equipment has neutral current on it and since the audio cables also share this same path this current is imposed upon the signal to the amps which are design to amplify 60hz very well.

You would have to track down this current, find it and eliminate it. Not an easy chore I might add. This will also happen when equipment is fed from two differant sub panels that at least one of them only has a three wire feed to the main service.
Good Luck

 

[james wuebker]

Re: Help! PA system AC hum

I believe dereckbc is correct. If you want to make sure run a power cord from your power amp circuit to your mixer board and every thing that's hooked to your mixer. I bet the hum goes away. If this isn't it then try unplugging inputs on your mixer one at a time. It could be your snake to your board that is bad. But again I think it's a ground loop. Let us know want you find.
Jim

 

[peter d]

Re: Help! PA system AC hum

Thanks for the advice, guys.

Wayne, the monitor amp and main amp are both fed from the mixing board. The monitor signal goes through one of the snakes to the monitor amp which is located on the stage. I will see if I can relocate the monitor amp to the mixing desk, but more than likely I will have to do some rewiring or use the isolation transformers that you mentioned.

In the meantime, they will have to rely on the orange extension cord that they had in the first place. :roll:

 

[peter d]

Re: Help! PA system AC hum

Update:

I ran a cord from the monitor amp to the new outlet I installed, and the hum stopped immediately. So obviously, it was a loop. I suggested they move the monitor amp to the mixing desk, as that was the easiest solution.